Re: [PATCH] mm: vmscan: do not skip CMA while LRU is full of CMA folios

[Barry Song]

On Mon, Apr 8, 2024 at 8:06 PM <liuhailong@xxxxxxxx> wrote:
>
> From: liuhailong <liuhailong@xxxxxxxx>
>
> If the allocation flag isn't movable, commit 5da226dbfce3 ("mm:
> skip CMA pages when they are not available") skips CMA during direct
> reclamation. This generally speeds up the reclamation of non-movable
> folios. However, in scenarios with limited system memory and a majority
> of reclaimable folios in LRU being from CMA, this can result in prolonged
> idle loops where the system reclaims nothing but consumes CPU.
>
> I traced the process of a thread entering direct reclamation,
> and printed the relevant information of the sc(scan_control).
> __alloc_pages_direct_reclaim start
> sc->priority:9 sc->nr_skipped_cma:32208  sc->nr_scanned:36  sc->nr_reclaimed:3
> sc->priority:8 sc->nr_skipped_cma:32199  sc->nr_scanned:69  sc->nr_reclaimed:3
> sc->priority:7 sc->nr_skipped_cma:198405 sc->nr_scanned:121 sc->nr_reclaimed:3
> sc->priority:6 sc->nr_skipped_cma:236713 sc->nr_scanned:147 sc->nr_reclaimed:3
> sc->priority:5 sc->nr_skipped_cma:708209 sc->nr_scanned:379 sc->nr_reclaimed:3
> sc->priority:4 sc->nr_skipped_cma:785537 sc->nr_scanned:646 sc->nr_reclaimed:3
> __alloc_pages_direct_reclaim end duration 3356ms
>
> Continuously skipping CMA even when the LRU is filled with CMA
> folios can also result in lmkd failing to terminate processes. The
> duration of psi_memstall (measured from the exit to the entry of
> __alloc_pages_direct_reclaim) becomes excessively long, lasting for
> example a couple of seconds. Consequently, lmkd fails to awaken and
> terminate processes promptly.

If you're planning to send a newer version, it's best to include
the reproducer here.

>
> This patch introduces no_skip_cma and sets it to true when the number of
> skipped CMA folios is excessively high. It offers two benefits: Rather
> than wasting time in idle loops, it's better to assist other threads in
> reclaiming some folios; This shortens the duration of psi_memstall and
> ensures timely activation of lmkd within a few milliseconds.
>
> Signed-off-by: liuhailong <liuhailong@xxxxxxxx>

I believe this patch tackles a niche scenario where CMA is configured
to be large.

Acked-by: Barry Song <baohua@xxxxxxxxxx>

> ---

This is evidently version 3 of your previous patch titled "[PATCH v2] Revert "
mm: skip CMA pages when they are not available".[1]

It's advisable to provide a brief explanation of the changes made from v2
and include a link to v2 here.

[1] https://lore.kernel.org/linux-mm/CAGsJ_4wD-NiquhnhK_6TGEF4reTGO9pVNGyYBnYZ1inVFc40WQ@xxxxxxxxxxxxxx/

>  mm/vmscan.c | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fa321c125099..2c74c1c94d88 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -114,6 +114,9 @@ struct scan_control {
>         /* Proactive reclaim invoked by userspace through memory.reclaim */
>         unsigned int proactive:1;
>
> +       /* Can reclaim skip cma pages */
> +       unsigned int no_skip_cma:1;
> +
>         /*
>          * Cgroup memory below memory.low is protected as long as we
>          * don't threaten to OOM. If any cgroup is reclaimed at
> @@ -157,6 +160,9 @@ struct scan_control {
>         /* Number of pages freed so far during a call to shrink_zones() */
>         unsigned long nr_reclaimed;
>
> +       /* Number of cma-pages skipped so far during a call to shrink_zones() */
> +       unsigned long nr_skipped_cma;
> +
>         struct {
>                 unsigned int dirty;
>                 unsigned int unqueued_dirty;
> @@ -1572,9 +1578,13 @@ static __always_inline void update_lru_sizes(struct lruvec *lruvec,
>   */
>  static bool skip_cma(struct folio *folio, struct scan_control *sc)
>  {
> -       return !current_is_kswapd() &&
> +       bool ret = !current_is_kswapd() && !sc->no_skip_cma &&
>                         gfp_migratetype(sc->gfp_mask) != MIGRATE_MOVABLE &&
>                         folio_migratetype(folio) == MIGRATE_CMA;
> +
> +       if (ret)
> +               sc->nr_skipped_cma += folio_nr_pages(folio);
> +       return ret;
>  }
>  #else
>  static bool skip_cma(struct folio *folio, struct scan_control *sc)
> @@ -6188,6 +6198,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>                         vmpressure_prio(sc->gfp_mask, sc->target_mem_cgroup,
>                                         sc->priority);
>                 sc->nr_scanned = 0;
> +               sc->nr_skipped_cma = 0;
>                 shrink_zones(zonelist, sc);
>
>                 if (sc->nr_reclaimed >= sc->nr_to_reclaim)
> @@ -6202,6 +6213,16 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>                  */
>                 if (sc->priority < DEF_PRIORITY - 2)
>                         sc->may_writepage = 1;
> +
> +               /*
> +                * If we're getting trouble reclaiming non-cma pages and
> +                * currently a substantial number of CMA pages on LRU,
> +                * start reclaiming cma pages to alleviate other threads
> +                * and decrease lru size.
> +                */
> +               if (sc->priority < DEF_PRIORITY - 2 &&
> +                   sc->nr_scanned < (sc->nr_skipped_cma >> 3))
> +                       sc->no_skip_cma = 1;
>         } while (--sc->priority >= 0);
>
>         last_pgdat = NULL;
> --
> 2.36.1
>

Thanks
Barry